Optical emission spectroscopy (OES), also termed atomic emission spectroscopy (AES), is a technique for the elemental analysis of samples and is particularly useful, for example, in the analysis of solid, metallic samples. The present invention relates to OES wherein a spark (herein used to refer to any electrical spark, arc or discharge) is used to rapidly vaporise a sample and excite elements in the vapourised sample, i.e. so-called spark OES. Light is emitted by the excited elements of the sample as transitions occur from an excited state to a lower energy state. Each element emits light of discrete wavelengths characteristic of its electronic structure, which are also termed spectral lines. By detecting the spectral lines, OES can provide a qualitative and quantitative determination of the elemental composition of the sample. A spark optical emission spectrometer thus includes a spark generator for exciting the elements in the sample to emit light, an optical system for dispersing the emitted light into discrete wavelengths, a detection system for detecting the light intensity of the dispersed light and a data storage and processing system for storing and processing signals from the detection system representing the light intensity. To build up sufficient data for determination of the composition, a succession of sparks is typically employed and the resulting data generated from the sparks is accumulated for processing.
A spark generator for producing a sequence of sparks for exciting a sample in OES should preferably produce sparks having a stable energy output and high degree of reproducibility for high accuracy of measurements.
Conventional analogue spark generators, in which sparks are generated in an unmodulated way by the discharge of a capacitor through a resistance and an inductance (RLC circuit), do not permit a high amount of control over the current waveform or profile of the spark and hence the reproducibility is low. Accordingly, the accuracy of measurement of components in a sample is adversely affected. The current waveform of an analogue spark source is generally characterised by a relatively slow rise of the spark current (compared to the digital sources described below) to a broad peak before a gradual fall or decay of the current in an exponential manner over a longer time period. It has been found that this type of unmodulated current profile is not well suited to the analysis of trace elements in samples. Although it may be better for analysis of alloying elements in metals than trace elements, even in that case the analogue generated spark still leads to the poor measurement accuracy mentioned above due to poor spark reproducibility.
So-called digital spark generators, e.g. as described in EP 396 291 B1, which generate modulated sparks are known and these have sought to address some of the above issues. In that reference is described a spark generator including means for measuring the spark current during the spark, comparing the spark current to a reference current and adjusting the spark current to a pre-determined value dependent on the reference current. The sampling rate for the reference comparison is said to be 50-200 kHz. The reference current is stored on a computer as part of a program for the spark current waveform. The modulated current waveform described in the prior art generally has a single initial peak of high amplitude (high current) and of relatively short duration followed by a longer lasting modulated decay of low current, which resembles something of a current plateau. The high amplitude peak may be a factor of 5 more intense than the current plateau. Such a waveform is shown schematically in FIG. 4 of EP 396 291 B1. The initial high current peak is described as being mainly for use in the evaporation of the sample and the longer lasting current is for exciting the atoms in the vapourised sample. It has been found that this type of current profile is better than the profile of the analogue spark source for detecting trace elements but is not so well suited to the analysis of alloying elements in metal alloy samples.
In JP 8-159973 A and EP 318 900 A2, another type of spark source is described in which two portions of current are produced. A high current portion is produced first which is a single peak and a low current portion is produced second which comprises two or three peaks of progressively lower intensity. These spark sources suffer from a lack of control due to the use of passive circuitry. In particular, the amplitudes and durations of current peaks are fixed by different values of capacitors and inductors. In EP 84566 A is disclosed a spark source employing a decaying oscillatory current source again employing passive circuitry (i.e. a resonant LC circuit) and therefore also suffers from a lack of control. The current envelope simply decreases along an exponentially decaying curve with peak frequency being determined by the resonant frequency of the circuit.
An additional problem with OES instruments employing charge coupled device (CCD) detectors as light detectors is that such detectors can exhibit deterioration in their response over time. It would be desirable to reduce the rate of such deterioration.
In view of the above background, the present invention has been made.